Make JS SDK Reference Easy to Find in Documentation Page

[krapie]

Description:

Make the JS SDK Reference easy to find on the documentation page.

Why:

We recently found out that many new contributors seem to struggle with finding the API reference page. This is because Yorkie's API Reference page is located at the bottom of the documentation page.

The API reference page is a very informative documentation page that many developers look at to find detailed information about how to use library (service) APIs.

Especially for the JS/TS SDK, it is crucial because developers need to know what type that Yorkie API uses.

Therefore, we need to find a better way to provide an API reference page in the documentation section.

(This issue is somewhat related to #95)

[krapie]

One approach that @mojosoeun suggested is to place the API reference page in the menu tap on the documentation page, like Firebase.

[hackerwins]

I'm also sympathizing with this issue. Feel free to send a PR for this.

[krapie]

@banma1234 Somewhat related to your previous suggestions about documentation of JS SDK.